1. Field of the Invention
The present invention relates to an apparatus and method for estimating the velocity of a mobile terminal. More particularly, the present invention relates to an apparatus and method for estimating the velocity of a mobile terminal in a mobile communication system.
2. Description of the Related Art
Mobile communication systems were developed to provide a mobile terminal (also called a mobile station (MS)) with mobility. The mobile communication system forms a channel between a base station (BS) and the MS to guarantee the mobility of the MS, such that it performs voice and data communication over the channel. The MS provides the mobility, such that a wireless signal generated from the MS does not always transmit an output signal at a fixed position. Specifically, the MS can constantly transmit the wireless signal at different positions, such that an environment and a path of a wireless channel are continuously changed. The MS moves with its user, such that it can transmit a wireless signal in a stationary state, or can transmit the wireless signal while in motion at a low or high speed.
As previously discussed, channel environments change in the mobile communication system, and a transmission path of a wireless signal also changes due to the change in an MS's position, such that the mobile communication system must receive data in consideration of the changed channel environments, the changed MS's position and the speed of the MS, and so on. Specifically, the mobile communication system estimates a transmission channel of the MS, and extracts data according to the estimated result. Therefore, provided that channel estimation of the MS is not correctly performed, the mobile communication system cannot extract correct data. Specifically, if the mobile communication system cannot estimate a correct channel, it is unable to extract data.
The channel estimation method of the mobile communication system will hereinafter be described. The mobile communication system transmits data to the MS over a forward link from the BS to the MS. In this case, the BS transmits a traffic channel signal and a pilot signal in order to allow the MS to perform channel estimation. The MS transmits data to the BS over a reverse link transmitted to the BS. In this case, the MS transmits a reverse pilot signal to allow the BS to estimate a reverse channel. Upon receiving the reverse pilot signal from the MS, the BS performs channel estimation using the received reverse pilot signal. The BS decodes a traffic signal received from a corresponding MS on the basis of the channel-estimated value. In this manner, if the decoding of the traffic signal is performed, data reception performance can be improved.
However, the Doppler shift occurs in a reverse pilot channel and a reverse traffic channel according to the velocity of the MS. Due to the Doppler shift, real channel estimation performance may deteriorate. Specifically, the real channel estimation performance changes in proportion to the shifting degree of a received signal. Also, the Doppler shift effect results in different values according to the velocity of the MS. Therefore, the BS must remove the Doppler shift at individual velocities of the MS to fully remove the Doppler shift effect and must also perform channel estimation at individual velocities of the MS, such that the BS requires a channel estimator at each velocity of the MS in order to remove the Doppler shift effect and perform channel estimation.
A method for estimating such velocities of the MS in a mobile communication system will hereinafter be described. The mobile communication system has divided the velocity of the MS into several velocity zones, and has a previously-designed optimum channel estimator for providing optimum channel estimation performance at each velocity zone. In this case, a plurality of channel estimators are used according to different velocities of the MS, such that a category of the channel estimator to be used must first be determined. In order to select the category of the channel estimator, a velocity estimator is required to estimate the velocity of the MS on the basis of a received signal. There are two methods for implementing the velocity estimator, i.e., a first method for adapting an autocorrelation function of a received signal in a time domain, and a second method for adapting a Discrete Fourier Transform (DFT) in a frequency domain.
As described above, the velocity estimator using the DFT in a frequency domain requires a procedure for estimating the Doppler spectrum using Mdft-point DFTs. However, if all the Mdft-point DFTs are calculated to estimate the shape of the Doppler spectrum, unnecessary resources may be wasted. Furthermore, the higher the velocity of the MS, the wider the Doppler spectrum bandwidth. Therefore, the number of channel estimation coefficients used according to widened Doppler bandwidths increases, and the number of frequency indexes to be calculated to estimate the Doppler spectrum also increases.